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Nissan’s variable compression turbo (VC-Turbo) engine has a multilink mechanism that continuously adjusts the top and bottom dead centers of the piston to change the compression ratio and achieve both fuel economy and high power performance. Increasing the exhaust gas recirculation (EGR) rate is an effective way to further reduce the fuel consumption, although this increases the exhaust gas condensation in the cylinder bores, causing a more corrosive environment. When the EGR rate is increased in a VC-Turbo engine, the combined effect of piston sliding and exhaust gas condensation at the top dead center accelerates the corrosive wear of the thermal spray coating. Stainless steel coating is used to improve the corrosion resistance, but the adhesion strength between the coating and the cylinder bores is reduced.

The application of both high strength gear steels and shot peening technology has succeeded in strengthening automotive transmission gears. This technology, though, improves mainly the fatigue strength at the tooth root, but not the pitting property at the tooth face. Therefore, demand has moved to the development of new gear steels with good pitting resistance. In order to improve pitting resistance, the authors studied super carburizing which is characterized by carbide dispersion in the case, especially processed with a plasma carburizing furnace. Firstly, the influence of the carburizing temperature and carburizing period on the carbide morphology was investigated and the optimum carburizing conditions were determined. Secondly, the fatigue strength and pitting resistance was evaluated using carbide dispersed specimens.

Excellent fuel economy and high performance have been urgent in Japanese automobile industries. With increasing engine power, many of the power train components have to withstand higher loads. Differential pinion gear being one of those highly stressed parts, excellent fatigue and shock resistance have been demanded. At first the fundamental study on the fatigue and impact crack behavior of carburized components was studied and the new grade composed of 0.18%C-0.7%Mn-1.0%Cr-0.4%Mo was alloy designed. Furthermore, Si and P is reduced less than 0.15 and 0.015%, respectively aiming at the reduction of intergranular oxidation and improved case toughness. The differential gear assembly test has proved that the new grade shows three times as high impact strength as that of conventional steel, SCM418, and almost the same as that of SNCM420 containing 1.8%Ni.

A new high-performance and lightweight TiA1 intermetallic compound exhaust valve has been developed. The TiA1 valve can improve power output and fuel economy by contributing higher engine speeds and a reduction in valvetrain friction. It was achieved by developing a Ti-33.5A1-0.5Si-1Nb-0.5Cr (mass%) intermetallic compound, a precision casting method for TiA1 that provides a low-cost, high-quality process, and a plasma carburizing technique for assuring good wear resistance on the valve stem end, stem and face.

The cold forging process is one of the most popular in the manufacture the automotive parts such as gears and shafts, cold forging saves material and machining costs by near-net shape the principle of forming. However, abnormal austenite grain growth sometimes occurs when the cold forged parts are heated for surface carburizing without a prior normalizing process. The size of the coarse grains can be large, sometimes ASTM Grain Size Number -2 to -4. The abnormal grain growth may cause post-carburizing distortion and is harmful to both fracture toughness and fatigue strength of the parts [1]. The purpose of our research was to develope new steels which would keep the fine grains during the carburizing treatment without normalizing. First, we studied the influence of elements on the grain growth property of case hardening steels and Naiobum (Nb) was selected as the element to control the grain growth. Secondly, we developed an ultra fine grain steel containing a small amount of Nb.

In recent years, the temperature of automobile exhaust gas is on a rising trend due to lowering pollutant emissions and improving fuel economy, and exhaust gas temperature reaches as high as 1173K in the case of diesel engine cars. Against this background, Ni-resist D-5S cast iron has been chosen extensively as a turbine housing material for the diesel engine cars. But, Ni-resist D-5S has become a material of great cost volatility due to high Nickel content of 35 mass%, which price is expensive and unstable. On the contrary Ferritic cast steels, which possesses favorable thermal fatigue properties and good material cost stability, are considered to be promising substitutions for the Ni-resist D-5S. However conventional ferritic cast steels have relatively high melting points, which cause poor castability.

As trends of automobile engine exhaust gas temperature are reducing emissions, the material for the exhaust components have been changed from ductile irons to ferritic cast alloys or stainless steel, further to austenitic cast alloys for higher performance engines. The current austenitic alloys, however, have thermal fatigue failure over 1273K. The authors developed excellent thermal fatigue resistant austenitic cast alloys, by investigating the effects of alloying elements on strength and thermal expansion, which correlate with thermal fatigue property. Developed alloys are expected to apply to exhaust components at gas temperatures over 1273K.

A free machining Pb-free steel was developed with shape controlled sulfide (SCS) for use in automobile applications such as rocker arms and crankshafts. This free-machining steel is characterized by its improved chip breakability, for which a technique that adds very small quantities of Ca and Ti to control sulfide shape was specifically applied. It was confirmed that this free-machining steel can offer almost equivalent machinability and equivalent or higher fatigue strength by comparison with Pb-added steel for use in rocker arms or crankshafts with S and Ti modifications.

A new variable compression turbo (VC-Turbo) engine, which has a multi-link system for controlling the compression ratio from 8:1 to 14:1, requires high axial force for fastening the multi-links because of high input loads and the downsizing requirement. Therefore, it was necessary to develop a 1.6-GPa tensile strength bolt with plastic region tightening. One of the biggest technical concerns is delayed fracture. In this study, quenched and tempered alloy steels were chosen for the 1.6-GPa tensile strength bolt.

It was found that pitting resistance of gears is strongly influenced by resistance to temper softening of carburized steel. The investigation about the influence of chemical compositions on hardness after tempering revealed that silicon, chromium and molybdenum are effective elements to improve resistance to temper softening and pitting resistance. Considering the production of gears, molybdenum is unfavorable because it increases hardness of normalized or annealed condition. Developed new steel contains about 0.5 mass% of silicon and 2.7 mass% chromium. The new steel has excellent pitting resistance and wear resistance. Fatigue and impact strength are equivalent to conventional carburized steels. Cold-formability and machinability of the new steel are adequate for manufacturing gears because of its ordinary hardness before carburizing. The new steel has already been put to practical use in automatic transmission gears. Application test results are also reported.

For the purpose of saving natural resources and energy, the requirements of vehicle weight-saving have been increasing continuously. As for Automotive Suspension Coil Spring, its weight-saving has been achieved by increasing the design stress. Since the increase of design stress requires higher fatigue life and sag resistance, the strength of spring is usually increased. However, in case of the conventional spring steel, the high strength over σB=1900MPa can dramatically reduce the corrosion fatigue life of spring, to decrease the reliability of spring at the actual usage. In this paper, newly developed spring steel material, satisfying higher strength and corrosion fatigue life simultaneously, is proposed, and its application of Automotive Suspension Coil Spring under the appropriate spring manufacturing processes in introduced.

Application of microalloyed steel to automobile parts is becoming increasingly common in Japan. However, fatigue properties of actual automotive forged parts with slight notches on their surface have not been fully clarified. In this work, the fatigue properties of microalloyed steel were studied using test specimens and also actual automotive parts. The results indicated that microalloyed steel with an optimal microstructure showed higher notch fatigue resistance than quenched-tempered steel. The improvement of material technology and the application of microalloyed steel have not only served to bring product costs down, but have paved the way for part weight reductions. Lightweight connecting rods for the newly developed Nissan engines have been produced, contributing to improved engine performance.

The influence of chemical compositions and forging conditions on mechanical properties of forged bainitic steels were studied. Manganese and chromium are useful to produce bainite structure while carbon and vanadium are good to control the mechanical properties of the steels. One of the compositions is 0.25 % C - 2.1 % Mn - 0.7 % Cr - 0.15 % V of which tensile strength is 1000 MPa and impact value (2 mm U type notched specimen) is 50 J/cm2 for 100 mm diameter bars. Bainitic steels have lower fatigue limit in the case of smooth specimen than ferrite-pearlite microalloyed steels but have higher fatigue limit in the case of notched specimen.

Based on fundamental research about the influence of chemical composition on rolled bar hardness, hardenability, case hardenability, cold formability, and mechanical properties, a new case hardening steel has been developed which can be cold forged without spheroidizing annealing. The steel contains boron and the Si and Mn contents are less than conventional steels. The steel shows fatigue strength equivalent to the conventional steels and better toughness and machinability.

In recent years, the use of aluminum die cast parts in automobile manufacturing has increased due to greater demand for automotive weight reduction. For even wider application, it is necessary to reduce manufacturing costs and improve product quality. Finite element method (FEM) analysis suggested that a new material, featuring 50% improved thermal conductivity within the working temperature of the die compared to the conventional 5% chromium hot work tool steel AISI-H13 (H-13), would decrease thermal stress on the die surface and lower the maximum surface temperature. As a result, the reduced stress should increase the die service life with respect to heat checks. At the same time, the reduced surface temperature should increase the cooling rate of die cast products, which will in turn improve product quality due to finer structure formation.

Although shot peening is an old technology, it has been revived in the Japanese automotive industry as a means to enhance the fatigue durability of steel components. Particular emphasis is on the application of “hard shot peening”. “Hard shot peening” is a high intensity peening technology which results in a higher magnitude of compressive residual stress and, therefore, greater fatigue resistance than conventional shot peening. The first area of development was in high performance carburizing steels suitable for hard shot peening. Desirable traits were enhanced by reducing the carburizing anomalies resulting from intergranular oxidation and by the enhancing case toughness. Further improvement of fatigue resistance has been accomplished by dual peening, first with hard shot followed by smaller diameter steel shot at a lower intensity. This paper also describes the development of long life shot media for hard shot peening.